grief over losing a spouse, or beginning dementia?

grief over losing a spouse, or beginning dementia?

Not sure of this subject, but I couldn't find any info anywhere else. My mom died about 8 months ago, my dad is 83, decent health, but his grief is substantial. (they were married 62 yrs.) Recently, I have noticed forgetfulness, also he says he "talks" with my mom, which I understand, I also am a widow, however, he says when he does he doesn't understand why she doesn't talk back. He knows she is gone, this bothers me. any thots?